The aeration of waste liquid media, including for example domestic sewage and industrial wastewaters, using submerged aeration devices, is an old art. A common aeration device is the fine pore diffuser. It transfers oxygen to wastewater by producing in the wastewater a discharge of small bubbles of a treatment gas such as air or other oxygen-containing gas.
In a fine pore diffuser, the component which actually produces the bubbles is a porous medium, sometimes referred to as a diffusion element, through which aeration gas received in the diffuser is forced into the wastewater. Porous media of various types have been used for this purpose.
MPDEs (multi-pore diffusion elements), the porous media employed in fine pore diffusers of the MPDE type, have been used in various forms for many years. These elements have been made for example by sintering of ceramic grains in the form of square or rectangular plates, as was popular from the 1920s through the mid-1950s, and to a lesser extent in the late 1950s and 1960s. An MPDE diffuser in which the element was formed of grains sintered in a dome shape was introduced in the mid-1940s, and enjoyed growing popularity in the 1950s, 1960s and 1970s. Diffusers with flat, disk-shaped elements formed of sintered ceramic or plastic grains were used in the 1970s and became popular in the 1980s. Voids between the sintered grains provide gas passages through which aeration gas may flow and discharge from an upper surface of the diffuser as small gas bubbles. Membrane fine pore diffusers, formed from sheet-like webs of synthetic elastomeric material, became popular in the 1980s. These webs, which may be of circular, rectangular, square or other shape, as viewed in plan view, have many small gas passages extending through them for discharging small bubbles.
Fouling of submerged fine pore diffusers is an old problem. Organic and/or inorganic foulants tend to progressively foul the diffusion elements at the point of release of aeration gas into the wastewater, and may also foul the air passages internally. Such fouling can impair the uniformity of gas distribution from aeration devices, and can increase the pressure differential required to drive the aeration gas through the aeration devices at a given flow rate.
The use of gas cleaning to confront the fouling problem received limited attention during the 1930s, 1940s and early 1950s. Early efforts concentrated on the cleaning of MPDEs. In 1939, W. M. Franklin cleaned the diffusers in several tanks of a wastewater treatment plant in Charlotte, N.C., introducing the cleaning gas through the same piping system which normally supplied air to the plenums for the diffusers. Based on his relatively few applications of cleaning gas at Charlotte, Franklin reported that the method was successful. See W. M. Franklin, "Purging Diffuser Plates With Chlorine," Water Works & Sewerage, June 1939, pages 232-233.
Further trials were made by R. B. Jackson at Jackson, Mich. The initial runs reportedly gave satisfactory results. See R. B. Jackson's article entitled "Maintaining Open Diffuser Plates With Chlorine," Water Works & Sewerage, September, 1942, pages 380-382. However, the last attempt reported in the article failed completely and inexplicably.
Franklin's and Jackson's articles were followed by trials at major wastewater treatment plants in Chicago and New York. In these trials it was found that the technique failed to clean the diffusers. Also, it was disclosed that the attempts at gas cleaning in Chicago and New York had actually impaired the diffusers by increasing their permeability and making them more difficult to clean by other methods. See the "Manual of Practice No. 5," Federation of Sewage and Industrial Wastes Associations, Champaign, Ill., 1952, pages 60-61.
In 1951, E. P. Coombs, inventor of the ASL (Activated Sludge Limited) dome diffuser and then managing director of ASL, spoke at a meeting of the Institute of Sewage Purification. His remarks included a warning against the use of gas cleaning on the grounds that it could clean some but not all of the pores of the diffusion elements and thus produced only a very temporary effect. In a guide to cleaning of MPDEs which was apparently first circulated in January 1953, ASL made clear that use of gas cleaning in their diffusers was definitely not recommended.
From the early 1950s forward through 1980, there was a dearth of reports of new activity with respect to gas cleaning of MPDEs. The 1971 edition of Manual of Practice No. 5 did nothing to explain away or dilute the previous unfavorable reports. ASL did not back away from their contrary position, but rather insisted that removal of the diffusion elements from the tanks and burning them in a kiln was the only truly satisfactory way of cleaning them. A variety of experts wrote articles and surveys on cleaning methods for MPDEs without any mention whatsoever of gas cleaning.
On Sep. 29 and Nov. 4, 1980, Frank L. Schmit, David T. Redmon, and Lloyd Ewing filed U.S. patent applications Ser. Nos. 191,974 and 203,834. The latter application issued May 10, 1983 as U.S. Pat. No. 4,382,867 entitled "In Place Gas Cleaning of Diffusion Elements" and disclosed a method of cleaning MPDEs in place with cleaning gas while submerged in liquid media by applying said cleaning gas intermittently or continuously to said diffusion elements between predetermined limits of DWP (dynamic wet pressure) or BRP (bubble release pressure) through flow regulation means and plenums for the diffusion elements. On Nov. 5, 1980, Schmit, the President of WPCC (Water Pollution Control Corporation), gave a speech to the EPA/WWEMA (U.S. Environmental Protection Agency/Water and Wastewater Equipment Manufacturers' Association) Innovative Alternative Seminar in Chicago, Ill., in which he discussed WPCC's in-situ gas cleaning process and its effectiveness.
Initially, commercializing the WPCC process was difficult. Initial resistance on the part of potential customers, based mainly on concerns about the efficacy of the process, was consistent with the unfavorable reports reviewed above. However, the WPCC process and apparatus were successfully demonstrated in several plants, and developed a good record with respect to efficacy. Eventually, WPCC's gas cleaning process was recognized as effective, and, by 1992, WPCC had licensed about 150 plants for use of its patented gas cleaning process.
The Cummings patent, U.S. Pat. No. 5,051,193, entitled "Wastewater Treatment Process," and issued Sep. 24, 1991, to Paul W. Cummings, Jr., discloses processes for cleaning MPDEs with liquid acid solutions. The stated objectives of the Cummings processes are to provide better penetration of blocked pores, to reduce the quantity of acid used, and to extend the period of acid-element contact. Cummings discloses two alternative processes requiring a series of manipulative steps.
In what we refer to as Cummings' "back-flow mixture" process, an acid gas under pressure is made to fill the aeration piping network, the elements, and their plenums. The pressure in the network is then reduced to allow wastewater from the tank to back-flow into and through the pores of the elements into the plenums and piping to mix with the acid gas. Pressure is then reinstated in the network to force the resultant liquid acid into the pores. There is then a quiescent period of up to several hours, in which there is no significant flow of aeration gas into the wastewater while the acid reportedly "reacts" with the foulants. Then, pressure may again be decreased, to allow further back-flow, and then reinstated to force more liquid acid through the pores. The pulsation and quiescent periods are repeated as many times as necessary.
During the foregoing steps, which may consume a number of hours, the flow of aeration gas into the wastewater is negligible, compared to the aeration gas requirements of normal wastewater aeration treatment. Finally, sufficient pressure is maintained to force more acid through the pores and into the tank, followed by continuous flow of aeration gas at normal rates. This process requires an investment of considerable time, during which the aeration gas flow is interrupted.
A second alternative procedure disclosed by Cummings, herein called the "external mixture" process, employs an acid solution. Prepared above ground, it is made to fill the aeration piping network, the elements, and the plenums. A slight pressure is maintained to prevent back-flow of wastewater from the tank and to keep the acid solution in contact with the foulants. After a quiescent period during which the acid solution reportedly reacts with the foulants for the desired period of time, there may be pulsing and additional periods of quiescent reaction as used in the latter stages of the back-flow mixture process.
Here again, aeration gas flow may be interrupted for hours. Finally a normal flow of aeration gas is re-established, and more acid solution is driven out through the pores of the elements.
At present, wide-spread use of the Cummings methods has not developed. The method has been available for a few years, but there are very few operating installations. The explanation may lie in the fact that the method is subject to a number of limitations, shortcomings, and adverse perceptions.
One factor which may have discouraged widespread use of the back-flow mixture process is the number of steps and controls which can be involved, some of which may involve delicate adjustment or balancing of pressures and flows. These include initiation of acid gas flow; controlling the rate of acid gas flow and stopping at proper time to provide proper amount of gas in piping and plenums to achieve proper liquid acid concentration upon subsequent admixture of gas with liquor from tank; reduction of aeration gas flow sufficiently to cause back-flow; reinstating aeration gas flow to flood elements while controlling aeration gas flow to prevent complete expulsion of acid from piping, plenums and elements, controlling pressure within the piping and plenums during quiescent period to prevent further back-flow and expulsion of acid from element inward into plenum; repeating two previous steps as necessary; and reinstating aeration gas flow in such a manner as to produce pulsation.
A concern about the back-flow mixture process arises from the possibility of back-flow of sludge into the element. Depending on the height of the water above the diffusers, substantial driving force could be developed to force smaller solids into the pores, fouling the diffuser. It is not apparent whether the foulants will be pushed out again during cleaning or when the aeration gas flow is re-established. See "Aeration--A Wastewater Treatment Process," WPCF (Water Pollution Control Federation) Manual of Practice No. FD-13, page 56.
Another difficulty, which arises in the use of the Cummings external mixture process, involves the sheer volume of liquid acid required. A typical grid contains hundreds to a few thousand diffusers, and a typical plant has multiple grids. A grid of 1000 dome diffusers with 4" diameter distributor piping would require over 1000 gallons of liquid acid to fill the diffusers and the distributor piping. Also, 1000 gallons of liquid acid represents about eighteen 55-gallon drums, weighing approximately 460 pounds each. Moving these drums about the tank site will require hard labor or mechanized drum-handling equipment. Elimination of drum handling will require expenditures for large acid storage tanks and their attendant accessories and safety devices.
An additional concern with both of the Cummings processes arises from the fact that they involve keeping the distribution system partially filled with liquid acid while aeration gas is passing through it. It is anticipated that the passage of aeration gas over and through the liquid acid will strip acid gas from the liquid and convey this acid gas through the diffusers. The question thus arises as to whether, in conducting the Cummings processes, a sufficient amount of acid gas would be conducted through the diffusers so that the diffusers would be cleaned by the resultant cleaning gas-aeration gas mixture instead of or in addition to the liquid acid mixture. In acid gas cleaning systems, most of the cleaning occurs in the first 20 minutes of acid gas addition. Therefore, in order that the liquid acid used in the Cummings process might perform most of the cleaning, the question arises as to whether it may be necessary, in order to clean primarily with liquid acid according to Cummings' concept, to fill the distribution system with liquid acid in considerably less than 20 minutes.
Employing the above example of a grid of 1000 dome diffusers with one foot distribution spacing, application of the external mixture method to fill the diffusers and the distributor piping with 1000 gallons of liquid acid in the space of, for instance, 15 minutes would require pumping 67 gallons per minute of liquid acid. High rate pumping results in increased safety concerns. The high volume rate of acid pumping and the rapid switching between drums during the pumping process would increase the possibility of personnel being exposed to spills and splashing of the acid solution. Here again, provision of large acid storage tanks with their attendant accessories and safety devices, could prove necessary to attain acceptable safety levels.
Another issue which may surface upon intensive use of the Cummings methods is whether it can be conveniently be determined when the diffusers have been cleaned. In line with the recommendations in the above-identified Schmit et al patent, Cummings suggests monitoring the condition of the diffusion elements, prior to cleaning, through the use of certain measuring devices which are capable of sensing the DWP (Dynamic Wet Pressure) across the elements. In gas cleaning, one can continue to monitor DWP during cleaning. However, DWP cannot be sensed during the quiescent conditions employed during liquid cleaning according to the Cummings patent. It may also happen that, even when there is gas flow during liquid acid cleaning as suggested by Cummings, that DWP measuring systems will not give accurate indications as to whether the elements have been fully cleaned until most if not all of the residual liquid acid has been driven from the system and the plant has returned to normal, stable operation. This could require a period of hours. If this is the case, then whenever treatment with a given batch of acid has been completed, there will be a delay in determining whether the elements have been fully cleaned, and whether or not a fresh batch of acid will have to be prepared for use in a subsequent cleaning step.
When soaking elements with liquid according to the Cummings processes, there is no appreciable aeration gas flow for extended periods. Interruptions of several hours are likely, during which the amount of treatment of wastewater BOD is greatly reduced. This is a distinct disadvantage when the plant is operating near its capacity or in an overloaded condition.
If, during use of the Cummings system-liquid-flooding process, one were to repetitively alternate between the quiescent periods taught by Cummings and the supplying of some amount of aeration gas to the wastewater, such as to dislodge foulants, or to provide the microorganisms in the wastewater with some oxygen or to maintain anoxic conditions suitable for phosphorous or nitrogen uptake, the onset of aeration gas flow will drive significant amounts of liquid acid cleaning agent from at least the upper portions of the aeration gas supply pipes and drive it out of the diffusers into the wastewater. Much of the liquid acid driven out of the diffusers in this manner will not therefore have an opportunity for extended contact with the gas passages in the diffusion elements. This acid will represent a waste in the sense that it must be replaced in order to begin another quiescent period.
The use of liquid acid solution for cleaning aeration devices in-situ as currently known in the art raises concerns about efficiency, complexity, back-flow of solids, repeated or extended interruption of aeration gas flow, safety, and cost. The present invention is aimed at reducing these difficulties.